In recent years, the fields of applications of polypropylene resins are growing widely because the polypropylene resins achieve an excellent balance among mechanical strength, heat resistance, processability and price, and also have excellent properties such as easy incinerability and easy recyclability.
Similarly, expanded beads-molded articles formed from uncrosslinked polypropylene resin beads are extensively used as packaging materials, construction materials, heat insulating materials and the like, since the expanded beads-molded articles are imparted with characteristics such as shock absorbing properties and heat insulation properties, without impairing the excellent property of polypropylene resins.
In recent years, the expanded beads-molded articles formed from uncrosslinked polypropylene resin beads are demaded to be lightweight and high-rigidity, particularly in the field of automobiles. Thus, investigations have been conducted on the use of high rigidity polypropylene resins as the base resin of expanded beads. High rigidity polypropylene resins tend to have higher melting points as rigidity increases. Therefore, in order to in-mold molding expanded beads formed of a high-rigidity polypropylene resin as a base resin, high steam pressure is required. Particularly, when the melting point of a polypropylene resin is 145° C. or higher, the steam pressure required for in-mold molding exceeds the pressure-resistant performance of conventional molding machines. Accordingly, when conventional molding machines are used, it is difficult to apply a sufficient steam pressure, and only those molded articles in which the fuse-bonding between expanded beads is insufficient, could be obtained. Therefore, in order to perform in-mold molding of high-rigidity expanded polypropylene resin beads having a melting point of 145° C. or higher, a special molding apparatus which is capable of enduring high steam pressure is needed. Furthermore, in the case of expanded beads formed of a high-rigidity polypropylene resin, there has been a problem that an enormous amount of steam is required for the use during molding.
Extensive investigations have been conducted in relation to expanded polypropylene resin beads so as to address such problems and to obtain an expanded beads-molded article capable of being molded within the range of the pressure-resistant performance of conventional molding apparatuses. For example, first, Patent Literature 1 has suggested expanded polypropylene resin beads which are formed from a particulate foam of a first polypropylene resin having a melting point of 140° C. or higher, and a foam of a second polypropylene resin closely adhering to the surface of the particulate foam, in which the melting point of the second polypropylene resin is lower by 2° C. to 10° C. than the melting point of the first polypropylene resin, and the second polypropylene resin has a specific surface area. However, although the expanded beads can be molded at a low steam pressure, the coating layer portion which corresponds to the second polypropylene resin expands and bursts during in-mold molding, the rigidity such as compressive strength of the resulting expanded beads-molded article is low.
Furthermore, Patent Literature 2 has suggested expanded beads which are each composed of a core layer that is formed from a crystalline thermoplastic resin and is in an expanded state, and a coating layer that is formed from an ethylene polymer having a lower melting point than the thermoplastic resin and is substantially in an unexpanded state. However, a molded article obtained by using the expanded beads has a problem that heat resistance is low, as proved by low flexural properties under heating conditions.
The Applicant of the present application intended to provide expanded polypropylene resin beads, with which an expanded beads-molded article having rigidity and heat resistance can be obtained, as described in Patent Literature 3, and defined the upper limit of the thickness of the coating layer in the resin beads. However, although the expanded beads can be molded at a low steam pressure, it is necessary to set the calorific value of the high-temperature peak of the expanded beads to be large, in order to increase rigidity such as compressive strength of the resulting expanded beads-molded article. As a result, since the secondary expanding power at the time of heat molding is insufficient, a so-called pressure molding method by which in-mold molding is carried out while a high internal pressure is applied to the expanded beads, may be necessary in order to make up for the insufficient expanding power, or the appearance of the resulting expanded beads-molded article may have many voids.
On the other hand, the Applicant of the present application has suggested a shock absorbing material having excellent rigidity such as compressive strength, which is formed by in-mold molding expanded beads having a modified surface capable of low temperature molding, as described in Patent Literature 4. However, there still are problems to be addressed for an improvement, such as the effluent treatment of organic peroxides used at the time of surface modification, and there is also a demand for a substitute.